The present invention generally relates to components for the turbine sections of gas turbine engines. More particularly, this invention relates to a gas turbine engine nozzle segment and a process for producing such a nozzle segment to exhibit improved durability and aerodynamic performance.
Components located in the high temperature sections of gas turbine engines are typically formed of superalloys. Though significant advances in high temperature capabilities have been achieved, superalloy components must often be air-cooled and/or protected with a coating to exhibit a suitable service life in certain sections of gas turbine engines. For example, components of the turbine, combustor, and augmentor sections that are susceptible to damage by oxidation and hot corrosion attack are typically protected by an environmental coating and optionally a thermal barrier coating (TBC), in which case the environmental coating is termed a bond coat that in combination with the TBC forms what may be termed a TBC system.
FIG. 1 represents a nozzle segment 10 that is one of a number of nozzle segments that when connected together form an annular-shaped nozzle assembly of a gas turbine engine. The segment 10 is made up of multiple vanes 12, each defining an airfoil and extending between outer and inner platforms (bands) 14 and 16. The vanes 12 and platforms 14 and 16 can be formed separately and then assembled, such as by brazing the ends of each vane 12 within openings defined in the platforms 14 and 16. Alternatively, the entire segment 10 can be formed as an integral casting. When the nozzle segment 10 is assembled with other nozzle segments to form a nozzle assembly, the respective inner and outer platforms of the segments form continuous inner and outer bands between which the vanes 12 are circumferentially spaced and radially extend. Construction of a nozzle assembly with individual nozzle segments is often expedient due to the complexities of the cooling schemes typically employed. The nozzle segment 10 depicted in FIG. 1 is termed a doublet because two vanes 12 are associated with each segment 10. Nozzle segments can be equipped with more than two vanes, e.g., three (termed a triplet), or with a single vane to form what is termed a singlet.
As a result of being located in the high pressure turbine section of the engine, the vanes 12 and the surfaces of the platforms 14 and 16 facing the vanes 12 are subjected to the hot combustion gases from the engine's combustor. As previously noted, in addition to forced air cooling techniques, the surfaces of the vanes 12 and platforms 14 and 16 are typically protected from oxidation and hot corrosion with an environmental coating, which may then serve as a bond coat to a TBC deposited on the surfaces of the vanes 12 and platforms 14 and 16 to reduce heat transfer to the segment 10. Environmental coatings and TBC bond coats are often formed of an oxidation-resistant aluminum-containing alloy or intermetallic whose aluminum content provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) provides protection from oxidation and hot corrosion, and in the case of a bond coat promotes a chemical bond with the TBC. Environmental coatings and TBC bond coats in wide use include alloys such as MCrAIX overlay coatings (where M is iron, cobalt and/or nickel, and X is yttrium or a rare earth element), and diffusion coatings that contain aluminum intermetallics, predominantly beta-phase nickel aluminide and platinum-modified nickel aluminides (PtAl). MCrAIX-type overlay coatings may be overcoated with an aluminide diffusion coating to further promote oxidation resistance as taught in commonly-assigned U.S. Pat. No. 5,236,745.
Because TBC life depends not only on the environmental resistance but also the strength of its bond coat, bond coats capable of exhibiting higher strength have been developed, a notable example of which is a material commercially known as BC52 and disclosed in commonly-assigned U.S. Pat. No. 5,316,866. BC52 is an MCrAIX-type overlay coating material with a nominal composition of, by weight, about 18% chromium, 10% cobalt, 6.5% aluminum, 2% rhenium, 6% tantalum, 0.5% hafnium, 0.3% yttrium, 1% silicon, 0.015% zirconium, 0.06% carbon and 0.015% boron, the balance nickel. Overlay environmental coatings and bond coats are typically applied by physical vapor deposition (PVD), particularly electron beam physical vapor deposition (EBPVD), and thermal spraying, particularly plasma spraying (air, low pressure (vacuum), or inert gas) and high velocity oxy-fuel spraying (HVOF). To promote the adhesion of a TBC, bond coat materials such as BC52 are deposited to have a very rough surface finish, e.g., about 400 microinches (about 10 micrometers) Ra or more as sprayed. For this reason, BC52 bond coats for plasma sprayed TBC's have been deposited by thermal spraying a coarse BC52 alloy powder to obtain the desired as-deposited bond coat surface roughness, and do not undergo further processing to smooth their surfaces. As a result of the thermal spray deposition process, the molten powder particles deposit as “splats,” resulting in the bond coat having irregular flattened grains and a degree of inhomogeneity and porosity.
The air-cooled nozzle segments of the high pressure turbine (HPT) stage 2 nozzle assembly currently used in the General Electric LM2500 industrial and marine turboshaft gas turbine engine are cast from the nickel-base superalloy known as René 80 (R80). A TBC is not required for the HPT stage 2 nozzle assembly, but the surfaces of the nozzle segments are protected with a cobalt-based MCrAIX-type overlay coating commercially known as BC22. The BC22 environmental coating is deposited and processed to have a very smooth surface finish, e.g., about 60 microinches (about 1.5 micrometers) Ra or less, in order to promote the aerodynamics of the nozzle assembly. Two processing routes have been employed, depending on whether the nozzle segments are doublets (as represented in FIG. 1) or singlets. If a singlet, the cast R80 nozzle segment undergoes drilling to form cooling holes, after which the holes are masked and the BC22 coating is applied by air plasma spraying (APS). To achieve a surface finish of 60 microinches or better, the coated casting undergoes shot peening and tumbling, after which singlet castings are brazed together to form doublets, which undergo aluminiding before being installed in the engine. If a doublet, the difficulty of depositing a uniform coating by plasma spraying necessitates that the cast R80 nozzle segment first undergo plating to deposit the BC22 coating. Thereafter, the coated casting undergoes shot peening and tumbling, after which the cooling holes are drilled and the casting undergoes aluminiding.
While the BC22 environmental coating material has performed well in the LM2500 application, improved coating durability, including oxidation and corrosion resistance, would be desirable, particularly for higher operating temperatures.